1. Field of the Invention
This invention generally relates to a high-speed magnetic levitation transportation system, and more particularly to a superconducting magnet for levitating and propelling a MAGLEV vehicle along a guideway.
2. Description of the Related Art
Referring to FIG. 1, a superconducting magnet 100 includes six low-temperature superconducting (LTS) coils 105 operating in DC mode surrounding respectively six leg portions 110 of a laminated iron core 115. The leg portions of the core project from a core base 120, which is mounted along a side of a MAGLEV vehicle in such a manner that alternating north and south magnetic poles surfaces 125, located at an end of each core leg, are suspended a predetermined distance away from and in opposing relation to a ferromagnetic rail 130 of a vehicle guideway.
In operation, DC excitation current supplied to the LTS coils from a constant current power source causes the coils to induce primary magnetic fields in their respective core poles. The core poles, as a result, become magnetically attracted to a vehicle guideway, counterbalancing the loading of the MAGLEV vehicle to cause the vehicle to levitate off the surface of the rail by a distance proportional to the strength of the magnetic fields induced in the cores.
MAGLEV superconducting magnets of the type just described have a number of drawbacks. One rather obvious drawback is the sophisticated cryostats these magnets must employ in order to fulfill the cooling requirements of the LTS coils. These cryostats, unlike higher temperature systems, must be equipped with on-board refrigerators that provide a continuous supply of low-temperature refrigerant for maintaining the LTS coils in a superconducting state, which, in some cases, may be at temperatures as low as 4 K. Because of the low-temperature requirement, these cryostats must be more thoroughly insulated, compared with higher temperature cryogenic systems, in order to protect the LTS coils from being compromised by heat loads leaking into the interior of the cryostat from the outside environment. The use of sophisticated cryostats of this type adds considerably to the cost and complexity of building, operating, and maintaining the superconducting magnet.
Another drawback centers around the inability of LTS coils to perform adequately under AC conditions. In practice, excitation current supplied to an LTS coil cannot be changed at a rate faster than 1 Hz, otherwise the coil will lapse into a non-superconducting state and the MAGLEV system will fail. LTS coils therefore must at all times be operated in DC mode.
Having to operate the LTS coils in DC mode is undesirable for a number of reasons. The first concerns vehicle stability and the inability of LTS coils to correct deviations in the air gap between the core poles and rail.
Regardless of whether the MAGLEV transport vehicle is at rest or in motion, the distance between the core poles and rail varies with changes in vehicle loading, wind loading, and vehicle pitch angle. In order to maintain vehicle stability, rapid correction of the pole-to-rail gap back to a desired length (e.g., 4 to 5 cm.) is imperative. LTS coils, however, are unable to make the rapid corrections required because, as previously discussed, the excitation current supplied to the coil cannot be changed faster than a rate of 1 Hz without causing the coil to lapse into a non-superconducting state.
To compensate for the slow reaction time of the LTS coils, fast-reacting non-superconducting control coils must be used to correct these deviations. Six control coils of this type, labeled 140, are included in the magnet shown in FIG. 1. Control coils 140, operating at a rate as high as 20 Hz, are included around the ends of the core legs for making rapid, fine-tuned adjustments to the pole-to-rail gap whenever the gap deviates from its desired length.
In operation, excitation current supplied to the control coils induces a secondary magnetic field in the core which, when added to the primary field already induced in the core by the superconducting coil, increases the forces of attraction between the core poles and rail to correct the size of the gap. Control coils 140 may be activated to adjust the total excitation in the core poles in either buck or boost mode.
Having to employ control coils to compensate for the shortcomings of the LTS coils increases the power and hardware requirements of the magnet, which, as in the case of the cryostat, translates into increased construction and operating costs.
Another reason DC operation is undesirable is the susceptibility of the LTS coils to being corrupted by external fields generated by the control coils. These external fields are highly undesirable because they can cause the LTS coils to lapse into a non-superconducting state. In order to neutralize the effects of external fields on the LTS coils, the superconducting magnet must be equipped with a control system, which modulates the current supplied to the control coils in such a manner so as to ensure that the LTS coils remain in DC mode.
Control systems of this type are disadvantageous because, one, they provide only a limited degree of control and, two, they, like the control coils themselves, add to the complexity and cost of operating and maintaining the magnet.
A need therefore exists for a superconducting magnet for a MAGLEV system which overcomes the problems realized by superconducting magnets which use LTS coils operating in DC mode, and further which can be built, operated, and maintained for a fraction of the cost of such magnets.